Electrical circuit for a wheel-acting force measuring device

Abstract: The invention provides a wheel-acting force measuring device designed to remove the brake torque (torsional torque) component mixed in a shearing stress detecting device. The four sensor segments a, b, c, d in the form of strain gauges of the stress detecting sensor are logically constructed to form four bridge circuits, the output from each bridge circuit being fed into an arithmetic circuit for finding the value of b/(a+b) - c/(c+d) = A (shearing strain equivalent) and an arithmetic circuit for finding the value of b/(a+b) - d/(c+d) = B (brake torque component equivalent), the outputs from the two arithmetic circuits being fed into an arithmetic circuit for finding the value of A - αB = C (shearing stress with brake torque component removed) so as to remove the brake torque component mixed in the shearing stress detecting device.

Description: BACKGROUND OF THE INVENTION The present invention relates to a wheel-acting force measuring device for measuring forces acting on a wheel, such as a road surface friction force and a road surface friction coefficient, which can be used as a component of either an antilock brake system (ABS) for preventing the locking of wheels on application of the hard brake to the vehicle or a traction control system (TCS) for preventing excessive slippage of wheels during acceleration. The present applicant has previously proposed, in EP-A-504731 acting force measuring device comprising a hole formed in or adjacent an axle, a stress detecting sensor, such as a road surface friction detecting sensor or a road surface friction coefficient detecting sensor, fixedly embedded in said hole directly or through a spacing agent such that said stress detecting sensor is disposed on the stress center axis, the detection signal from said stress detecting sensor being processed in a signal processing circuit composed of a bridge circuit. With the wheel-acting force measuring device described above, the complicated structure of the axle and vehicle body sometimes causes the stress detecting sensor and sensor segment to deviate from the ideal positions on the stress center axis of the brake torque, resulting in the brake torque (torsional torque) component, besides the shearing stress, being mixed in the bridge output of the signal processing circuit. Therefore, in such case, the correct stresses, such as the road surface friction force and road surface friction coefficient, cannot be detected and hence an antilock brake device or traction control system of high precision cannot be obtained. Thus, it is necessary to eliminate cross talk due to brake torque. SUMMARY OF THE INVENTION The present invention has been accomplished with the above in mind and is intended, to automatical by remove the brake torque component mixed in the stress detecting sensor output when stresses in the wheel are to be detected. This object is met by the independent claims. Preferred embodiments are disclosed in the dependent claims. A device according to the present invention is a shearing stress detecting device, comprising a stress detecting sensor having four sensor segments in the form of strain gauges attached to the front and back of a thin base sheet, said stress detecting sensor being installed on the stress center line of or adjacent an axle of a vehicle, said stress detecting sensor being constructed in the form of a bridge circuit whose sensor segments are adapted to be shiftly connected, the arrangement being such that after the brake torque component mixed in the shearing stress detecting device has been found, it is removed from the shearing stress detecting device by using said brake torque component as correction term. In this wheel-acting force measuring device, the connection of the sensor segments of the stress detecting sensor can be optionally changed to provide a setting adapted to preferentially selectively detect a desired stress, thus eliminating the brake torque component mixed in the shearing stress detecting device. According to a preferred embodiment a stress detecting sensor comprises four sensor segments in the form of strain gauges attached to the front and back of a thin base sheet, said stress detecting sensor being installed on the stress center line of or adjacent an axle of a vehicle, said stress detecting sensor being constructed in the form of a bridge circuit whose sensor segments are adapted to be shiftily connected, so that the side force can be detected by shifting it downward through 90 degrees (F-direction). In this wheel-acting force measuring device, the side stress can be detected by changing the connection of the sensor segments of the stress detecting sensor through 90 degrees, i.e., in the F-direction. According to a further preferred embodiment the four sensor segments a, b, c, d in the form of strain gauges of the stress detecting sensor are logically constructed to form four bridge circuits, the output from each bridge circuit being fed into an arithmetic circuit for finding the value of b/(a+b) - c/(c+d) = A (shearing strain equivalent) and an arithmetic circuit for finding the value of b/(a+b) - d/(c+d) = B (brake torque component equivalent), the outputs from the two arithmetic circuits being fed into an arithmetic circuit for finding the value of A - αB = C (shearing strain with brake torque component removed) so as to remove the brake torque component mixed in the shearing stress detecting device. In this wheel-acting force measuring device, the four bridge circuits logically constructed of four sensor segments forming the stress detecting sensor each detect the the shearing strain equivalent and the brake torque component equivalent, the outputs being calculated in the arithmetic circuits for finding these values, the output from the two arithmetic circuits being fed into the arithmetic circuit which subtracts the value of the brake torque component equivalent from the value of the shearing strain equivalent, this operation resulting in the brake torque component being removed in the logic block. According to a further preferred embodiment a stress detecting sensor body comprising a plurality of stress detecting sensors in the form of strain gauges attached to a common base sheet at suitable intervals is installed on the stress center axis of or adjacent the axle of a vehicle to form a shearing stress detecting device, the four sensor segments in the form of strain gauges of each stress detecting sensor of said stress detecting sensor body are logically constructed to form four bridge circuits so as to constitute a plurality of logic circuits corresponding to the number of the stress detecting sensors, the output from the arithmetic circuit of each logic circuit for finding the value of the shearing strain equivalent A and the output from the arithmetic circuit of each logic circuit for finding the value of brake torque component equivalent B are fed into a tendency deciding arithmetic circuit for the value of shearing strain equivalent and a tendency deciding arithmetic circuit for the value of brake torque component equivalent, the outputs from the two tendency deciding arithmetic circuits being fed into a comparison arithmetic circuit, thereby removing the brake torque component mixed in the shearing stress detecting device. In this wheel-acting force measuring device the shearing strain equivalent and the brake torque component equivalent are detected in a plurality of logic circuits corresponding to a plurality of stress detecting sensors attached to a single base sheet, the outputs from the logic circuits are fed into the tendency deciding arithmetic circuit for the value of shearing strain equivalent and the tendency deciding arithmetic circuit for the value of brake torque component equivalent, the tendencies are selected by arithmetic operations from the values of a plurality of shearing strain equivalents and the values of a plurality of brake torque component equivalents in said two tendency deciding arithmetic circuits, the selected outputs are fed into a comparison circuit, the brake torque component equivalents are removed from the shearing strain equivalents by arithmetic operations, and the stress detecting sensor located in the optimum position is automatically selected from the plurality of stress detecting sensors, so that the brake torque component mixed in the shearing stress detecting device is eliminated in the logic block. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a stress detecting sensor used in a wheel-acting force measuring device according to the present invention; Fig. 2 is a diagram of a bridge circuit of the sensor segments of stress detecting sensor Fig. 3 is an explanatory view showing changes in the resistance value of the sensor segments of a bridge circuit shown in Fig. 2; Fig. 4 is a diagram of a circuit diagram having a different relation of connection of the sensor segments; Fig. 5 is a diagram of a bridge circuit capable of changing the connection of the sensor segments; Fig. 6 is a block diagram showing an example of the hardware construction of a wheel-acting force measuring device described in Claim 3; Fig. 7 is a perspective view of a stress detecting sensor body used in a wheel-acting force measuring device described in Claim 4; Fig. 8 is a block diagram showing an example of the hardware construction of a wheel-acting force measuring device described in Claim 4; and Fig. 9 is an explanatory view for finding an imaginary line representing the stress center axis. DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiments of the invention will now be described with reference to the drawings. Fig. 1 is a perspective view showing an example of a stress detecting sensor according to the present invention. This stress detecting sensor F comprises four strain gauges a, b, c, d of metal resistance wire bonded, each two in cross form, to the front and back of a thin base sheet 1 of plastic material, such as epoxy resin, metal, silicone or the like, said base sheet being installed directly on the stress center axis of or adjacent an axle for a nondriven wheel of an automobile having a strut type suspension construction or being embedded in a hole, so as to detect stresses acting on a wheel, such as the road surface friction force and road surface friction coefficient. In the case where stresses are to be detected, the four strain gauges a, b, c, d are connected to form a bridge circuit D1, as shown in Fig. 2, so that a stress is derived in the form of a signal from the output terminals 2 and 3. When a strain is produced in the thin base sheet 1 of the stress detecting sensor F, the four sensor segments a, b, c, d (strain gauges) disposed on the front and back thereof changes such that if the direction of travel of the vehicle is as shown by the arrow in Fig. 1, the pre-stress-application state (3-1) with 10 and 11 for the front and back in Fig. 3 changes, in the case of a shearing strain, to the post-stress-application state (3-2) with 12 and 13 for the front and back whose changes are substantially the same. On the other hand, in the case of a strain produced by brake torque, since the sensor segments a, b, c, d are disposed in a symmetrical position with respect to the stress center axis, a torsional strain is produced in the thin base sheet 1, so that changes are opposite in pushing and equal in size, as shown at 14 and 15 in the state (3-3) in Fig. 3. Therefore, in the case of the bridge structure shown in Fig. 2, for a shearing stress, the output is obtained from the relations a = c and b = d, and for a brake torque (torsional torque) the output is zero from the relations a = c and b = d; thus, the bridge circuit delivers its output only for a shearing stress. However, this case where said brake torque (torsional torque) results in zero output takes place only when said stress detecting sensor F is disposed ideally in alignment with the stress center axis. In practice, because of the structure of the vehicle, it is extremely difficult from the standpoint of the technique of attaching said stress detecting sensor to dispose the stress detecting sensor in exact alignment with the stress center axis of or adjacent the axle. In many cases, such stress detecting sensor is disposed more or less deviated from the stress center axis, resulting in the brake torque (torsional torque) component being contained in the output from the circuit. If the bridge circuit is constructed by interchanging the sensor segments (strain gauges) d and c as shown in Fig. 4, then for a shearing stress, a = c and b = d with zero output, and for a brake torque (torsional torque), a ≠ c and b ≠ d, so that the brake torque component can be detected. Thus, by shiftly connecting the bridge circuit D1 shown in Fig. 2 and the bridge circuit D2 shown in Fig. 4 by means of mode change switches SW 1 and SW2 as shown in Fig. 5, two types of quantities can be detected using a single bridge circuit; more particularly, a stress with a brake torque component mixed therein as in the bridge circuit D1 shown in Fig. 2 can be detected and by changing the mode change switches, a brake torque component alone as in the bridge circuit D2 shown in Fig. 4 can be detected. Thus, the shearing stress without brake torque component can be obtained by subtracting the detected value of the brake torque alone from the detected value of the stress having brake torque component mixed therein. Fig. 6 shows an embodiment wherein shift connection of the bridge circuits D1 and D2 is constructed in a logic bridge manner. In this embodiment, four sensor segments a, b, c, d constitute independent bridge circuits 21, 22, 23, 24 which are connected to a common power source e, the output terminals of the bridge circuits are connected to arithmetic circuits 31 and 32 through amplifiers AP, thereby forming a logic bridge L. The arithmetic circuit 31 is used to find the value of the same arithmetic operation as in the bridge circuit D1 shown in Fig. 2, i.e., the value of b/(a+b) - c/(c+d) = A (shearing strain equivalent) and the arithmetic circuit 32 is used tu find the value of the same arithmetic operation as in the bridge circuit D2 shown in Fig. 4, i.e., the value of b/(a+b) - d/(c+d) = B (brake torque component equivalent), the outputs from these two arithmetic circuits being fed into the arithmetic circuit 33, where the value of A - αB = C (shearing strain with brake torque component removed) is found, whereby a detection signal with brake torque component removed is obtained. In addition, α is a coefficient which is empirically determined. Fig. 7 shows an embodiment of a stress detecting sensor body F comprising stress detecting sensors F1, F2, F3 each of which is the same as the stress detecting sensor F consisting of four strain gauges shown in Fig. 1 and which are integrally disposed with a suitable distance therebetween, said stress detecting sensor body being directly attached to or adjacent an axle or being embedded in a hole formed in or adjacent an axle, said stress detecting sensors F1, F2, F3 being constructed to form logic circuits L1, L2, L3 as shown in Fig. 8, each being the same as that shown in Fig. 6, the outputs from the arithmetic circuits of 31 and 32 of the logic circuits being fed into a tendency deciding arithmetic circuit 40 for the value of shearing strain equivalent and a tendency deciding arithmetic circuit 41 for the value of brake torque component equivalent, thereby reading the tendency of the value A of shearing strain equivalent and the tendency of the value B of brake torque (torsional torque) component equivalent. The outputs from the two tendency deciding arithmetic circuits are fed into an arithmetic circuit 42 for finding an imaginary iine (g in Fig. 9) indicating the ratio of the outputs G and H such that the signal component of the output value G of shearing strain equivalent is at a maximum. And by arithmetic operations it is possible to find an imaginary line g indicating the removal of the brake torque component. ADVANTAGES OF THE INVENTION According to the inventive wheel-acting force measuring devices desired stresses can be obtained by optionally changing the sensor segments of the stress detecting sensor and hence the stress with brake torque component removed can be easily obtained. A preferred embodiment of the inventive wheel-acting force measuring device has a particular advantage of being capable of detecting a stress in which the brake torque (torsional torque) component, which cannot be removed by the ordinary bridge arrangement because of the complicated construction of the vehicle and its axles, mixed in the shearing stress detection value, has been removed by arranging the sensor segments of the stress detecting sensor to form a logic bridge. Another preferred embodiment of the inventive wheel-acting force measuring device has a peculiar advantage of being capable of reading the tendencies of the shearing strain equivalent output and brake torque component, automatically selecting the optimum value so as to detect a stress with the brake torque component removed even in the case where because of the construction of the vehicle or axle, a stress detecting sensor cannot be installed on the stress center axis where the brake torque component is a at a minimum.

Claims: A wheel-acting force measuring device, which is a shearing stress detecting device, comprising a stress detecting sensor having four sensor segments in the form of strain gauges attached to the front and back of a thin base sheet (1), said stress detecting sensor being installed on the stress center line of or adjacent an axle of a vehicle and being constructed in the form of a bridge circuit, characterised by shift connection means for the outputs of the four sensor segments of the stress detecting sensor, said shift connection means being adapted to determine a brake torque component, means being provided to remove the brake torque component from a stress detecting sensor output. A wheel-acting force measuring device according to claim 1, wherein the shift connection means for the outputs of the four sensor segments of the stress detecting sensor is adapted to form a first bridge circuit from the four sensor segments to determine the stress detecting sensor output including the brake torque component and to form a second bridge circuit to determine the brake torque component, wherein the second bridge circuit is obtained from the first bridge circuit by interchange of two sensor segments. A wheel-acting force measuring device according to claim 2, wherein the shift connection means comprises two mode change switches (SW1, SW2) to interchange the two sensor segments. A wheel-acting force measuring device according to claim 1, wherein the four sensor segments having outputs a, b, c, d in the form of strain gauges of the stress detecting sensor are logically constructed to form four bridge circuits (21, 22, 23, 24), and the shift connection means is adapted so that the output from each bridge circuit being fed into an arithmetic circuit (31) for calculating b/(a+b) - (c+d) = A (shearing strain equivalent) and an arithmetic circuit (32) for calculating b/(a+b) - d/(c+d) = B (brake torque component equivalent), the outputs from the two arithmetic circuits being fed into an arithmetic circuit (33) for calculating A - αB = C (shearing stress with brake torque component removed) so as to remove the brake torque component mixed in the stress detecting sensor output, where α is a coefficient which is empirically determined. A wheel-acting force measuring device according to claim 3, wherein a stress detecting sensor body comprising a plurality of stress detecting sensors (F1, F2, F3) in the form of strain gauges attached to a common base sheet (1) at suitable intervals is installed on the stress center axis of or adjacent the axle of a vehicle to form a shearing stress detecting device, the four sensor segments in the form of strain gauges of each stress detecting sensor of said stress detecting sensor body being logically constructed to form four bridge circuits so as to constitute a plurality of logic circuits(L1, L2,L3)corresponding to the number of the stress detecting sensors, the output from the arithmetic circuit of each logic circuit for finding the value of the shearing strain equivalent A and the output from the arithmetic circuit of each logic circuit for finding the value of brake torque component equivalent B being fed into a tendency deciding arithmetic circuit (40) for the value of shearing strain equivalent and a tendency deciding arithmetic circuit (41) for the value of brake torque component equivalent, the outputs from the two tendency deciding arithmetic circuits being fed into a comparison arithmetic circuit (42), said comparison arithmetic circuit being adapted to remove the brake torque from the shearing stress.

Inventors: MIYAZAKI NAGAO, MIYAZAKI, NAGAO
Applicants: JAPAN ELECTRONICS IND LTD, JAPAN ELECTRONICS INDUSTRY, LTD.
Application year: 1994
Main classification: G01L1
IPC codes: G01L1, G01N19, B60T8, G01L5